Rotary voice coil actuators in disk drives typically employ upper and lower ball bearing assemblies 12 and 14, as shown in FIG. 1. Such assemblies are relatively low cost, and have experienced widespread usage. They manifest a frictional torque in accordance with the ball friction multiplied by a radius R extending from the center of rotation of an actuator shaft 16 to approximately the center of rotation of a nominal rotating ball 18 of the bearing assembly.
As disk drive actuator assemblies become smaller, the ratio of friction (both static and dynamic) to actuator inertia increases to the point where it becomes difficult for the actuator head position servo system to cause small displacements needed for single track seeking and in order to follow accurately a data storage track centerline. As concentric track densities (i.e. "tracks per inch" or "TPI") increase, motions of ever decreasing magnitude are required in order to carry out small distance seeks, and to maintain desired head position over the track.
Undesirable low frequency resonance incident to small motions of the ball bearings in actuators is attributable to distortions of the ball which result in a high stiffness (K) with very low damping (.eta.). During small motions, the actuator ball responds to an applied force by deforming elastically. The driving force is increased until the ball starts to roll and motion is realized. Unfortunately, by the time that the ball is driven out of its elastic deformation state and into rotary movement, excessive driving current has been applied, and the data transducer head is significantly mispositioned with respect to the desired corrected position. Initial deformation of the bearing ball results in high starting (i.e., static) friction ("stiction") and suggests that alternative bearing arrangements need to be considered.
Jewel bearings are known to provide a low friction coefficient at low frequencies, and have been employed in a wide variety of light load applications, particularly in clocks, watches, compasses, meter movements, etc. A variety of jewel bearing designs and applications are described in an article by Paul Baillio entitled "Jewel Bearings Solve Light Load Problems" in Machine Design, Oct. 26, 1989, pages 111-114. This article describes a technique of mounting a ball on the end of a shaft as one way to achieve higher thrust loads against a bearing jewel.
Pivot ball bearings are also known. Such cylindrical ball bearing assemblies comprise an outer, annular, fixed ball bearing race in which a plurality of highly polished steel balls are positioned. A cone-shaped pivot is loaded against the balls and thereupon defines an inner race. Pivot bearings of type BCF made by NSK, Japan, are available in a range of outer diameters of 3 mm to 16 mm, and define inner openings for cone pivots in a corresponding range of diameters of 0.81 mm to 8.11 mm. There are a number of drawbacks associated forming cones on actuator shafts including the costs associated with precision machining required, as well as surface smoothness and hardness of the machined cone bearing surface.
Despite the availability for many years of jewel bearings, and more recently of pivot ball bearing assemblies for use with pivot cones, a hitherto unsolved need has arisen for a reduced friction rotary actuator for a disk drive.